A critical distinction must be made for a modern libertarian in the US (I'm assuming this is US as that's where the "Defund the police" movement is currently prevalent). Others have covered the general libertarian ideologies, and this isn't intended to disagree with those assessments, in fact a key point is borrowed below. However, in general, modern libertarians in the U.S. tend to focus on reducing the federal government's impact on the day to day life of it's citizens, followed by reducing the state's impact, and lastly reducing the local government's impact, but with the expectation each level will indeed have a greater impact than the broader government above it. This is because the function of each should be limited far beyond the current state of the government. In that sense, most modern libertarians share many of the ideas of the founding fathers. To be clear this is intended as an addition to the ideals described as libertarian in the previous answers, not in conflict.
To borrow directly from @Phillip "The rights to life, liberty and property need protection, which requires a police force." - and as such a typical libertarian view would be to maintain a police force. The question then is re-framed as "How much policing is enough? How much is too much?".
The key distinction mentioned above is that, generally speaking, a modern libertarian in the US would support defunding the police at a federal and state level(local police funding, not state police), but would not necessarily support defunding the police at a local level. The core idea being the police (as most often referenced) are a local unit of authority meant to protect basic rights and liberties and nothing more.
"What about interstate crime?" - There are already numerous federal agencies designed to handle these situations, which local police could still effectively cooperate with.
"What about inter-county or inter-city crime?" - One wouldn't defund the state police, but remove the states funding to local police.
Additional funding isn't necessary and/or desired from a state and federal level for two main reasons:
- The duties of the police are limited and shouldn't require additional federal tax dollars to function appropriately. "But we should get the tax dollars if we can" - as a libertarian, one believes the tax dollars shouldn't be there to begin with, and you can't cut those taxes until they aren't spent on a program.
- Receiving funding from another government entity in-debts you to that entity. Once the money is in a budget it becomes expected, once it's needed for the entity to function as it expects, the entity supplying those funds has sway over the actions of the entity receiving the funds via a threat to no longer provide said funds.
The core idea here being that a separation of funding better facilitates a separation of control, which promotes the ideas of strictly necessary government, but no more.
It's worth mentioning that while federal aid only accounts for ~20% of police funding, the programs to provide surplus military equipment and similar tools to the police is a very real form of funding without directly providing $$$, and arguably one of the most detrimental to the police's image to the public, and to actually achieving their goals (only through the overuse of such equipment, there are no doubt times it's beneficial).
Funnily enough, there's one thing Libertarians have in common with Marxist, Communists, Liberals, Conservatives, even Anarchists - they only believe in providing a government that is strictly necessary to achieve an optimal society. The terms we use to describe political groups are simply guidelines of what a person in that group believes to be the optimal form of government. That's why the ideals associated with these terms and the terms themselves change consistently with time.
To attempt a summary, a modern day libertarian would support eliminating federal and state funding for local police, but not necessarily defunding on a local level. Like everyone else, they believe in funding the police to the exact level the police require to perform what they believe are the necessary duties of the officers. The term libertarian more accurately describes what the officers should enforce, not whether they should be there in the first place.
I skipped over state police to a high degree - felt the answer was too long already. But the ideas I mention should make the rest surmisable.
This stems mostly from personal discussion with self-described Libertarians combined with my own reading and political ideals - it's meant to provide a perspective but not be a definitive answer. Shockingly the sample of "personal discussion with self-described Libertarians" from my lifetime is relatively small compared to the population of the United States.
More personal note/not a direct answer:
I think what's most important is that you establish how you feel about defunding the police. This opinion can and should change as you research more, understand more, and consider other's viewpoints. That being said, there's no reason not to state what you believe now regardless of the assumed principles of a group you identify with. If your friend says your views aren't that of a Libertarian yet you describe yourself as one, simply tell them you generally agree with Libertarian ideas but don't adhere strictly to any political term to describe your overall personal political beliefs.